Opening for a routine over in A&E (8)
I believe the answer is:
aperture
'opening' is the definition.
(aperture is a kind of opening)
'a routine over in ae' is the wordplay.
'a' becomes 'per'.
'routine' becomes 'rut' (rut is a kind of routine**).
'over' is a reversal indicator.
'in' is an insertion indicator.
'rut' reversed gives 'tur'.
'per'+'tur'='pertur'
'pertur' going within 'ae' is 'APERTURE'.
'for' acts as a link.
(Other definitions for aperture that I've seen before include "Opening, gap" , "Hole, gap" , "Opening, to let light enter a camera perhaps" , "Opening (for a photographer?)" , "An opening that admits light" .)